Louis' Lunch is a restaurant in New Haven, Connecticut. It is considered the birthplace of the hamburger. It is known for serving hamburgers on toast with only cheese and tomatoes. It is also known for a lack of condiments (mustard, ketchup, mayo etc.) since they feel they interfere with the taste. They serve their burgers with potato salad.
Louis' Lunch is the oldest hamburger diner existing in America and one of the first to sell steak sandwiches. They have lingo in which food is to be ordered, and do not accept credit cards.